In general the function of a circuit breaker is to electrically engage and disengage a selected circuit from an electrical power supply. This function occurs by engaging and disengaging a pair of operating contacts for each phase of the circuit breaker. The circuit breaker provides protection against persistent overcurrent conditions and against the very high currents produced by short circuits. Typically, one of each pair of the operating contacts are supported by a pivoting contact arm while the other operating contact is substantially stationary. The contact arm is pivoted by an operating mechanism such that the movable contact supported by the contact arm can be engaged and disengaged from the stationary contact.
There are two modes by which the operating mechanism for the circuit breaker can disengage the operating contacts: the circuit breaker operating handle can be used to activate the operating mechanism; or a tripping mechanism, responsive to unacceptable levels of current carried by the circuit breaker, can be used to activate the operating mechanism. For many circuit breakers, the operating handle is coupled to the operating mechanism such that when the tripping mechanism activates the operating mechanism to separate the contacts, the operating handle moves to a fault or tripped position.
To engage the operating contacts of the circuit breaker, the circuit breaker operating handle is used to activate the operating mechanism such that the movable contact (s) engage the stationary contact(s). A motor coupled to the circuit breaker operating handle can also be used to engage or disengage the operating contacts. The motor can be remotely operated.
A typical industrial thermal-magnetic circuit breaker will have a continuous current rating ranging from as low as 15 amps to as high as 160 amps. The tripping mechanism for the breaker usually consists of a relatively quickly acting magnetic short circuit release, for larger overcurrents, including those of short duration, and a relatively slowly acting thermal overload release, for longer term and lower level overcurrents.
In the event of current levels above the normal operating level of the thermal overload release, it is desirable to trip the breaker without any intentional delay, as in the case of a short circuit in the protected circuit, therefore, an electromagnetic trip element is generally used. In a short circuit condition, the higher amount of current flowing through the circuit breaker activates a magnetic release which trips the breaker in a much faster time than occurs with the bi-metal heating. It is desirable to tune the magnetic trip elements so that the magnetic trip unit trips at lower short circuit currents at a lower continuous current rating and trips at a higher short circuit current at a higher continuous current rating. This matches the current tripping performance of the breaker with the typical equipment present downstream of the breaker on the load side of the circuit breaker.
The thermal overload release operates by means of a bimetallic element, in which current flowing through the conducting path of a circuit breaker generates heat in the bi-metal element, which causes the bi-metal to deflect and trip the breaker. The heat generated in the bi-metal is a function of the amount of current flowing through the bi-metal as well as for the period of time that that current is flowing. For a given range of current ratings, the bi-metal cross-section and related elements must be specifically selected for such current range resulting in a number of different circuit breakers for each current range.
It is known to provide for a thermal-magnetic circuit breaker to include a thermal trip adjustment capability, wherein the time and/or overcurrent tripping characteristics of the circuit breaker, and thereby the current rating of the circuit breaker, can be adjusted by a worker in the field. But only some field applications require thermal adjustment capability. Other field applications require a fixed, non-adjustable tripping means. It would be advantageous to provide for a thermal-magnetic circuit breaker to include an optional thermal adjustment feature, so that the adjustment capability may be removed, disabled, or enabled by the manufacturer. It would also be advantageous for a circuit breaker having an optional thermal adjustment feature to require changing or replacing of relatively few parts to remove, disable, or enable the thermal adjustment feature. It would further be advantageous for a circuit breaker having an optional thermal adjustment feature to utilize a single set of tooling for producing a housing and cover for the circuit breaker, and to utilize relatively inexpensive tool accessories such as inserts and plugs for adapting the tooling to produce circuit breaker housings and covers with and without a thermal adjustment feature.